SUMMARY/ABSTRACT Most tumors contain a cellular heterogeneity whereby a subpopulation of cells, commonly termed cancer stem- like cells (CSCs), are resistant to common therapies and hence drive tumor recurrence. It is now appreciated that preclinical and clinical trials need to evaluate the impact of the treatment being tested on CSCs. However, current isolation methods for CSCs have technical limitations that diminish the potential for rigor and reproducibility across laboratories and trial sites. Therefore, there is an urgent need to identify novel CSC isolation paradigms that are based on a more stable CSC feature. Altered iron (Fe) metabolism in cancer results in a net increase in Fe concentration within tumor cells as compared to non-neoplastic cells. More recently, CSCs from a variety of tumor types have been identified as the specific tumor cells with preferential Fe uptake and storage. In our proposed studies, we aim to take advantage of the magnetic moment that can be induced in Fe when an external magnetic field is applied to track/separate the CSCs from bulk tumor samples based on the differential magnetism that our preliminary studies indicate exists between CSCs and the rest of the tumor cells. Over the last fifteen years, our group has developed an instrument, referred to as cell tracking velocimetry (CTV), or single cell magnetophoresis (SCM), which quantifies the motion induced by a magnetic field on cells or particles on a single cell/particle basis. The CTV/SCM instrument has reached sufficient development that we are able to statistically quantify Fe content on a per cell basis on the order of 4107 Fe atoms per cell. This resolution is sufficient to not only quantify loss of hemoglobin in clinically approved red blood cell storage solutions during ex vivo storage but also separate cells based on this intrinsic magnetization. Our published data using the CTV/SCM system on GBM CSC has demonstrated clear differences in magnetic susceptibility for CSCs compared to the rest of the tumor. Further, we have conducted studies indicating that we can separate the CSCs from non-stem tumor cells magnetically. From this, we hypothesize that CSCs can be enriched from bulk tumor samples based solely on differential intrinsic magnetization. To test this hypothesis we will; 1) Improve the capabilities of the CTV/SCM system. 2) Demonstrate that individual cell types within a bulk tumor yield a unique paramagnetic profile. With the ultimate successful development of this technology, researchers and clinicians across multiple institutions/trial sites will have available an unbiased and reproducible method to evaluate the CSC population before and after treatment. With the known role of CSCs in tumor recurrence, this will directly inform the level of minimal residual disease for any therapy and hence guide therapeutic decisions at the individual patient level as well as inform overall efficacy of any novel treatment paradigm.